1. Field of the Invention
The present invention relates to a photosensitive planographic printing plate and a method of producing the same, and in particular to a photosensitive planographic printing plate most suitable as a multi-layer printing plate having a plurality of coating layers formed therein and a method of producing the same.
2. Description of the Related Art
In a production line for a photosensitive planographic printing plate, a web (aluminum continuous support) is fed in a longitudinal direction while one uneven side of the web is coated with a photosensitive coating solution, and this solution is dried to form a coating layer (photosensitive layer as dry film). Various attempts have been made to conduct this drying efficiently.
For example, Japanese Patent Application Laid-Open (JP-A) No. 7-89255 prescribes the ratio of the thickness of a photosensitive layer to the height of protrusions on a grained surface of a web, and after application of a coating solution onto the web, pressurized air is blown from a slit nozzle to the resulting wet coating, to permit the surface of the photosensitive layer to have a contour shape along the uneven surface of the web. Whether the photosensitive layer is a single or multiple layer is not particularly prescribed in JP-A No. 7-89255. In JP-A No. 6-317896, a planographic printing plate is heated such that the temperature of the printing surface is kept in a range of 50 to 130° C. by irradiation with far infrared rays in a drying process, thereby reducing the time required for the drying process to improve production efficiency.
In JP-A No. 6-63487, hot air is blown over a web, and then a heat roll is brought into contact with the web so as to make the contact area changeable, thereby controlling the dry state. In JP-A No. 8-318198, a heat roll is used in the latter half of a process of drying with hot air blown over a web. In this case, the temperature of the web is monitored, the lap width of the heat roll is regulated, and heating with the heat roll and blowing of hot air are simultaneously conducted.
As a photosensitive planographic printing plate in recent years, a direct printing plate is mainly used, and the use of a multi-layer printing plate is increasing in order to differentiate functions which are required of a photosensitive planographic printing plate. For continuous mass production of the multi-layer printing plate, a coating drying zone needs to be added to a conventional coating drying line. However, when such a coating drying zone is added, there arises a problem in that compact arrangement of facilities, reduction in running cost by effectively utilizing the quantity of heat supplied, and consistent quality of a web of continuously changing size, etc. are to be especially persued. This problem has been a task in general photosensitive planographic printing plates, as well.
When the thickness of a coating layer (coating) is uneven in a multi-layer-type CTP plate, particularly in a thin layer of 1.0 g/m2 or less, the sensitivity of the printing plate becomes uneven. Particularly, when there is a difference in film thickness between convex and concave portions of the grained surface of each layer, a noticeable difference in sensitivity occurs, which greatly influences the performance of the printing plate.
For example, when two layers of underlayer 84 and top layer 88 are formed on aluminum substrate 86 as shown in FIG. 5, the thickness d1 of the top layer 88 in top layer portion 88S over convex portion 92 of the aluminum substrate 86 is about 0.14 μm, while the thickness d2 of the top layer 88 in top layer portion 88T over concave portion 90 is about 1.0 μm. Accordingly, the excessively thin top layer portion 88S is poor in scratch resistance, while the excessively thick top layer portion 88T results in a dotted residual coating film because of failure to remove the coating by development treatment, thus hindering improvement in product yield. In the multi-layer-type CTP plate, therefore, there is also a problem in that, in order to improve the quality thereof, the top layer 88 must be provided with uniform thickness so that the functions of the top layer 88, which have been differentiated, are each successfully utilized.
This has serious implications for the multi-layer-type CTP plate mainly used in recent years. That is, an immediate solution is required for the above-described problem of the top layer in the multi-layer coating layer.
In a system of drying with air blown from an air slit nozzle as described in JP-A No. 7-89255, the speed of blown air is high. Accordingly, when this system is used in the first half of a drying process wherein a photosensitive solution film is thick, the state of the coating surface deteriorates. On the other hand, if the speed of blown air is decreased for securing the state of the coating surface, the effect of rapid drying is reduced. Thus, this system is unsuitable for drying the photosensitive solution film.
When the air slit nozzle is used in a production line wherein the size of a web (size of an aluminum substrate) is continuously changed, the quality thereof is significantly influenced by a slight difference in drying site. Further, a difference in drying speed occurs in the width direction. In short, application of the drying system using an air slit nozzle to a method of producing a planographic printing plate wherein drying starts from the edge of a web has not been put to practical use due to the difficulty in controllability.
It is difficult to solve the above problem even by drying methods described in other patent references.